1. Field of the Invention
The present invention relates to an extending circuit for memory and a transmitting-receiving device using the extending circuit for memory that are applied in digital data communication or the like.
2. Description of the Related Art
In digital data communication, data processing speeds on a transmission side and a reception side should be equal to each other in principle. Particularly in fields of mobile communication and the like, however, average data processing speeds on both sides are equal with each other, but the processing speeds on both sides are different for a short time. In such a case, FIFO (First In First Out) circuits are used in order to absorb the difference in the processing speeds. The FIFO circuits are first in first out circuits for literally outputting data according to an input order. As a memory capacity of the FIFO circuit becomes larger, a larger speed difference can be absorbed instantaneously. Enlargement of the memory capacity, however, causes a rise in the cost. Normally, a balance between the rise in the cost and the processing ability is considered according to applications or the like of the FIFO circuits, and the memory capacity is determined in a fixed manner. In the case where the application of the FIFO circuits is changed, some techniques for extending the memory capacity at a later time are disclosed (for example, see Japanese Patent Application Laid-Open No. 05-020864 (1993) as Patent Document 1 which relates to the invention of this application).
In the above-mentioned prior technique, in the case or the like where the memory capacity of the FIFO circuit is insufficient, it is difficult to extend the memory capacity later. As mentioned in the Patent Document 1, some techniques for extending the memory capacity later are disclosed, but various restrictions are placed on the extension of the memory capacity. It is thus difficult to enlarge the memory capacity of the existing FIFO circuits instantly and simply.